Internal pole piece arrangement for a magnetically-focused cathode ray tube



March 12, 1957 c. v. FOGELBERG ET AL 2,785,330 INTERNAL POLE PIECEARRANGEMENT FOR A MAGNETICALLY-FOCUSED CATHODE RAY TUBE Filed 001 19,1953 INVENTORS Glemcni 1 1i) Elbe/'9 Q Unitfid tates Patent INTERNALPOLE PIECE ARRANGEMENT FOR A MAGNETICALLY-FOCUSED CATHODE RAY TUBEClement V. Fogelberg and Edgar W. Morse, Chicago,

Ill., assignors to National Video Corporation, Chicago,lll.

Application October 19, 1953, Serial No. 386,964

11 Claims. (Cl. 313-84) This invention relates to an internal pole piececonstruction and arrangement for a magnetically-focused cathode ray tubeor the like.

In the foregoing connection the use of internal pole pieces forconcentrating the flux emanating from the magnet placed exteriorly ofthe tube at the tube axis, i. e. the electron beam axis, is well known.United States Letters Patent No. 2,619,607 shows some typicalarrangements of :such internally disposed pole pieces.

Since the space externally available along the neck of the tube issomewhat limited and is occupied by the deflecting yoke and ion trapmagnet, if any, the exterior magnetic focusing arrangement is limited asto extent and position. Preferably magnetic focusing should occur as farforward in the tube as practical construction will permit. Consequently,and since the deflection yoke must be positioned intermediate thejunction of the tube neck and tube body and the electron gun, themagnetic focusing means including the beam centering means isconfined'to a very short region of the neck.

In previous magnetic focusing systems which located the focusing devicenear the deflection system, interaction between the focusing field andthe deflection field was 1 unavoidable. In a tube having internal polepieces the .focus field is largely confined to the low-reluctancepolepiece region, and interaction between the focus field and deflectionfield is materially reduced. Shortening of the anode barrel of the gunfurther reduces field interaction.

in addition to energy requirements, consideration of the relativecentering motion available with different forms of pole piece isimportant. Centering of the beam isoften obtained by shifting the axisof the magnetic field by the use of an exterior shiftable pole piece.When a [device of'this kind is used the shielding effect of the polepieces tends to smooth the effect of shifting the outside pole to thepoint where insufficient centering motion is available. Longer gaplengths between the internal pole pieces tend to increase centeringmotion but increase the energy requirements for focusing. Thisdifficulty may be avoided by displacing the exterior focusing devicetoward the face of the tube. By such expedient there is provided fluxsuflicient for beam centering without substantial increase in the energyrequirements for focus ing and centering of the beam.

The present invention contemplates the provision of internal pole pieceswhich fulfill the above objective of positioning the point ofinteraction of the magnetic field and the beam as far forward in thetube as possible to efiect isolation of the magnetic field while stillconfining such point within the magnetic portion of the last electrode.Where, herein, We refer to the last electrode we intend to encompass theforemost electrode (G3) to whichmaximum beam accelerating potential isapplied and do notinclude other elements which may also occupy forwardpositions on the gun assembly but which are beam.

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A preferred mode of carrying the invention into practice is shown in thedrawing, in which:

Fig. 1 shows a portion of a cathode ray tube incorporating theinvention;

Fig. 2 is a detail, somewhat enlarged, of the pole piece unit apart fromthe electron gun; and

Fig. 3 is a schematic representation of a cathode ray tube of the typeto which the invention is adapted.

Regarded broadly, the invention contemplates the provision of a pair ofpole pieces of magnetic material carried on the forward end of asubstantially conventional electron gun. The rearward pole piececomprises a cupshaped part and a frusto-conical part, the smallerdiameter of which is forward. The forward pole piece is cup-shaped witha flat bottom. Essentially it is the frusto-conical part and the discpart, the gap formed therebetween and the forward positioning of the gapwhich constitute the gist of the improvement. The cupshaped portion ofeach pole piece is adapted to be supported by anon-magnetic sleevewhich, in effect, forms a part of the last accelerating anode. Thecup-shaped portion of each of the pole pieces is extended diametricallyto lie closely adjacent the wall of the tube neck in order to minimizethe annular air gaps intermediate the internal and external pole pieces,which latter are associated directly with the source of flux. Theinvention, as has already been alluded to, contemplates the provision ofan external source of magnetic flux of any suitable character and whichforms no part of the invention as such.

Turning to the drawing we have shown the glass neck 10 of a cathode raytube, e. g. a television picture tube, closed at the base, andsupporting in any customary manner an electron gun 12. This latterincludes a forward anode l3, referred to in this art as electrode G3constituting the forward electrode of a so-called slashed gun of awell-known type. Appropriate means 7 are included for deflecting thebeam of electrons emanating from the gun 12 in directions orthogonal tothe beam axis as is customary in the arta raster or image appearing onthe screen 8. Although electromagnetic beam-deflecting means are shownby way of example in Fig. 3 it will be understood that the'same maycomprise electrostatic means.

The conventional stop 16 for defining the circular cross section of theelectron beam prior to focusing thereof desirably includes a flange 17protruding beyond' the periphery of the anode 13 and serving as asupport for the pole piece structure to be described.

Such structure includes a rear pole piece 21 and a front pole piece 22mutually spaced and integrated by a sleeve 23. The pole pieces are ofmaterial from 0.010 to 0.020 in thickness and having the desired highmagnetic permeability, e. g. ordinary cold rolled steel, whereas thesleeve 23 is of non-magnetic material to avoid shunting of the flux.

Pole piece 21 comprises a cup-shaped part including a rim 25 and annulus26 which is secured to the flange 17 by spot welding or otherwise.Sleeve 23 fits snugly over and is secured to, a cylindrical portion 27of the pole piece 21, which latter continues in a frusto-conical portion28 terminating in a cylindrical portion 29.

Front pole piece 22 comprises an annulus 32, a cylindrical portion 33over which the sleeve 23 fits snugly and to which it is secured, and anannular disc 34 having its circular aperture 35 substantially equal indiameter to the internal diameter of the cylindrical portion 29. It willbe noted that the inter-pole space A is defined ata point toward thefront end of the pole piece assembly (Fig. 2) and thus sufliciently faraway (from the magnetic field used for ion separation located near theelectrode 12 to avoid interaction of the fields user respectively forfocusing and ion separation.

Any conventional getter assembly 41 may, if desired, be supported on thefore part of the pole piece assembly. To assist in locating the gunassembly coaxially with the tube neck, spring 42 or other customaryexpedient may be incorporated with the flange 32. However, since theseform no part of the invention they will not be elaborated upon.

Magnetic flux may originate from any suitable source. For example, onepresently preferred type comprises essentially a plurality of permanentmagnets 45', e. g. Alnico V, equipped with annular pole pieces 46 and 47having an aperture fitting as closely as practicable about the neck iiiof the tube. One commercial arrangement thereof which includes permanentmagnets, adjustment features and means for shifting the axis of thefield is dis closed in Patent No. 2,640,868. Any electromagneticfocusing device of known construction may be used equivalently.

Ideally the axial spacing of the pole pieces 45 and :7 corresponds withor is somewhat greater than the distance between the extremities of thepole-piece assembly, as seen in Fig. 2. Thus flux originating at themagnets 45 finds a path through the pole pieces 46, 22, 2t and 4-7 withthe usable flux concentrated across the gap A. In particular the rim 25will reduce the reluctance of the gap defined between it and the outerpole piece 47 thereby further to reduce the energy required.

Centering of the beam in a tube incorporating internal pole piecesrequires that displacement of the beam be accomplished as close aspossible to the face of the tube. If this requirement is not satisfiedand centering is done further back there exists the possibility that thebeam may be cut by the several apertures of the gun. By centering actionwe refer to the use of an eccentrically adjustable external pole pieceportion, for example, as shown in said Patent No. 2,640,863. Whencentering the beam by means of the character disclosed therein we havefound that maximum range of centering adjustment is attained when theforward pole piece is sub stantially a flat plate. Inter-rial pole piecearrangements which use a pair of opposed truncated cones, whilegenerally slightly more efiicient magnetically, do not allow adequatecentering motion. By constituting the internal pole pieces in accordancewith the principles of the invention the bundle of flux lines finds itsgreatest density in the gap A at a point sufiiciently forwaro in thetube to avoid the shortcomings incident upon the use of prior artconstructions. It will be comprehended from invention is equallyapplicab e to tubes including a pinrality of electron guns, e. g. thethree-gun tube of the RCA color television system. Hence, wherein theclaims we refer to a gun or gun means we intend to encompass assembliesother than those using a single gun. While we have shown a particularembodiment our invention, it will be understood, of course, that we donot wish to be limited thereto since many modifications may be made, andWe therefore contemplate by the no pended claims to cover "any suchmodifications as fall.

the foregoing that the .within the true spirit and scope of ourinvention.

We claim:

1. In combination with a cathode ray tube having electron gun means forproviding a beam of ClGCil'O-IIS for impingement on a luminescent,substantially planar, target area, said gun including a foremostcylindrical anode, means for traversing said beam in directionsorthogonal to the beam axis and magnetic flux-producing means externallyof the tube for focusing sm'd beam on the target surface, the tubeenvelope in the region of said magnetic means being non-magnetic;magnetic pole pieces within the envelope surrounding said beam toreceive flux through the envelope to provide aconcentrated bundle ofiretio 'fiuxliues' of predetermined strength and direction coactive withthe electron beam to focus the same at the target, one of said polepieces comprising a disc positioned with its principal plane normal tothe axis of the beam and having aperture for passage thereof and theother said pole piece comprising a frusto-conical shell and acylindrical portion unitary therewith at the larger diameter thereof andhaving an axis substantially coin cident with the beam axis, the end ofthe shell of smaller diameter confronting said disc to define a magneticgap therewith, said disk and cylindrical portion being secured withinanode for mutual axial alignment, said pole pieces being carried withsaid gap toward the forward end of the anode.

2. The combination in accordance with claim 1 wherein said disc and thelarger end of said shell are of substantially the same diameter as theinterior of the adjoining portion of the tube envelope.

The combination in accordance with claim 2 further characterized in thatthe external flux-producing means includes a pair of pole pieces ofannular form spaced apart along the beam axis closely fitting theexterior of the tube envelope and said disc and the larger end of saidshell are respectively substantially co-planar with the exterior annularpole pieces.

4-. The combination in accordance with claim 1 further characterized inthat the smaller end'of said shell also includes a hollow cylindrical.portion protruding forwardly of the tube to confront the disc and todefine the magnetic gap therewith.

5. The combination in accordance with claim 4- in which the diameter ofthe disc aperture and of the interior of said ho low cylindricalportion, are substantially equal.

6. In combination with a cathode ray tube having means for providing abeam of electrons for impingement on a luminescent, substantiallyplanar, target area, said gun including a foremost cylindricalnon-magnetic anode for accelerating said beam in its passage from thebeam-producing means toward the target area, means for traversing saidbearn in directions orthogonal to the beam axis and magnetiflux-producing means including annular elements axially spaced along thebeam and externally of the tube for focusing said beam on the targetsurface, the tube envelope in the region of said magnetic means beingnon-magnetic; magnetic pole pieces within the envelope surrounding saidbeam to receive flux through the envelope to provide a concentratedbundle of magnetic flux lines of predetermined strength and directioncoactive with the electron beam, one of said internal pole piecescomprising a disc carried in the forward end of the anode and spacedinwardly thereof, and having an aperture for passage of the beam, andthe other of said internal pole pieces comprising a frustoconical shellhaving its large end adjacent the rear end of said anode, the smallerend of said shell being spaced away from said disc to define a magneticgap therewith disposed within the forward half of the anode.

7. The combination in accordance with claim 6 wherein the smaller end ofsaid shell also includes a hollow cylindrical portion extendingforwardly and, confronting said disc.

8. The combination in accordance with claim 7 Where in the diameter ofthe disc aperture and of the interior of the smaller end of the. shellare substantially equal.

9. In combination with a cathode ray tube having a non-magnetic neck, abody portion diverging therefrom, a luminescent target area, an electrongun within said neck for providing a beam of electrons which includes asource of electrons and a foremost electron-accelerating electrode ofhollow cylindrical form, magnetic means external to said neck forproviding a bundle of flux lines within the zone encompassed by theelectrode for focusing the beam at the target area; pole means forconcentrating the flux iineswithin the electrode in a predeterminedconfiguration to provide the principal field of interaction of the beamand flux in the forward portion yrs of the electrode, said pole meanscomprising a disc normal to the beam having a central circular apertureand having its plane substantially at the foremost margin of theelectrode and a shell magnetically separated from said disc including afrusto-conical portion and a cylin drical portion extending forwardlyfrom the smaller end of the trusts-conical portion, the end of thecylindrical portion having an internal diameter substantially equal tothe diameter of the disc aperture whereby the flux available internallyof the neck is substantially entirely concentrated between the margin ofthe aperture and the free end of the cylindrical portion.

10. In combination with a cathode ray tube having electron gun means forproviding a beam of electrons for impingement on a luminescent,substantially planar, target area, said gun including a foremostcylindrical anode, means for traversing said beam in directionsorthogonal to the beam axis and magnetic flux-producing means externallyof the tube for focusing said beam on the target surface, the tubeenvelope in the region of said magnetic means being non-magnetic; aforward, magnetic pole piece including a cylindrical rim and aradially-disposed disc portion having a central aperture for passage ofthe beam; a rearward magnetic shell-like pole piece including afrusto-conical part, a cylindrical rim at the larger diameter of saidpart and a cylindrical rim at the smaller diameter of said part, saidpart and rims being coaxial with said aperture for passage of the beam;the respective cylindrical rim of said forward pole piece and of thelarger end of said rearward polepiece snugly fitting and being securedto the anode, the respective edges of said rims being substantially inalignment with the ends of the anode.

11. In combination with a cathode ray tube having electron gun means forproviding a beam of electrons for impingement on a luminiscent,substantially planar, target area, said gun including a foremostcylindrical anode, means for traversing said beam in directionsorthogonal to the beam axis and magnetic flux-producing means externallyof the tube for focusing said beam on the target surface, the tubeenvelope in the region of said magnetic means being non-magnetic; aforward magnetic pole piece including a cylindrical rim, a radiallydisposed disc portion having a central aperture and coterininousperipherally with one end of the rim and an annular portion coterminouson its inner circumference with the other end of said rim; a rearwardmagnetic pole piece including a frusto-conical portion, with its smallerdiameter facing forwardly, a first cylindrical portion extendingrearwardly from the larger diameter of said frusto-conical portion, anannular portion exending radially outwardly of the rear edge of saidfirst cylindrical portion, and a second cylindrical portion extendingforwardly of the smaller diameter of said frustoconical portion, the endof said second cylindrical portion defining with said disc aperture amagnetic gap, said two annular portions terminating closely adjacent theinternal surface or" the tube neck, said rim and first cylindricalportion fitting snugly within and supported by the anode, and theaxially disposed faces of said annular portions abutting the respectiveends of the anode.

References Cited in the file of this patent UNITED STATES PATENTS2,149,101 Ploke Feb. 28, 1939 2,212,206 Holst et al. Aug. 20, 19402,619,607 Steers Nov. 25, 1952 2,664,514 Reiches et a1. Dec. 29, 1953

